Conventionally, a number of studies have been carried out in which a carbon nano-structure material such as a carbon nanotube, a carbon nanohorn or the like is used as a conductor of a micro-electronic element, an electrode of FPD or the like, a microstructure, an adsorbent made of a material having high strength, or the like.
Such a carbon nanotube as described above has a cylindrical shape obtained when a graphite sheet were to be rounded in a cylindrical shape. A carbon nanotube can be classified into a single-wall carbon nanotube having a single sheet and a multi-wall carbon nanotube in which a plurality of sheets are overlapped like a plurality of nests. A single-wall carbon nanotube of well known type generally has diameter of 1 to 2 nm and length of a few μm. In a multi-wall carbon nanotube, the diameter of the outermost layer thereof is generally tens of nm, the diameter of the innermost layer thereof is a few to 10 nm, and the length thereof is generally a few μm.
On the other hand, a carbon nanohorn represents a structure in which the tip end of a single-wall carbon nanotube is closed in a conical shape. In practice, such a carbon nanohorn is obtained as a carbon nanohorn aggregate in which a plurality of carbon nanohorns are radially aggregated with the conical tip ends thereof being oriented outward so as to form a spherical body whose diameter is approximately 100 nm.
Among such carbon nano-structures as described above, a multi-wall carbon nanotube has been discovered in which, although the diameter of the outermost layer thereof is 10 nm or so, carbon nanotubes are densely stacked in the multi-wall carbon nanotube to the core portion thereof and diameter of the innermost layer as the smallest tube is approximately 0.4 nm. The diameter of approximately 0.4 nm of the innermost layer of this multi-wall carbon nanotube is the smallest of the diameters of the relevant carbon nanotubes that have ever been discovered. Further, the aforementioned diameter of approximately 0.4 nm of the innermost layer of the multi-wall carbon nanotube substantially corresponds to the diameter of a molecule of the smallest fullerene (C20). This means that the aforementioned diameter of the innermost layer has reached the ultimate smallness, a diameter smaller than which would not allow a carbon nanotube to exist in a stable manner. In other words, the multi-wall carbon nanotube having the aforementioned diameter of the innermost layer is a multi-wall nanotube in which tubes are densely stacked to a degree that no further inner tubes can be generated therein.
It is assumed that the above-described, densely stuffed multi-wall carbon nanotube has a characteristic that, in addition to being chemically stable like the conventional multi-wall carbon nanotube, the mechanical strength thereof has been significantly enhanced. Further, there has been revealed another interesting characteristic of the densely stuffed multi-wall carbon nanotube that the innermost carbon nanotube thereof having diameter of 0.4 nm has a metallic characteristic. The latter characteristic differs from the characteristic, observed in the conventionally obtained single-wall carbon nanotube whose diameter is 1 to 2 nm, that the conventional single-wall carbon nanotube may have either a metallic characteristic or a semiconductor-like characteristic, depending on the sequence of carbon atoms.
Thus, a carbon nano-structure, whose characteristic significantly changes by a slight change in the structure thereof, has potential as a material for various functions, which potential is not known yet. Further, it is expected that more carbon nano-structures of new types will be discovered in future.
The invention of the present application has been achieved in consideration of the above-described discoveries. An object of the present invention is to provide a acute tip, multi-wall carbon nanotube radial aggregate as a novel carbon nano-structure, which is useful as field emission electron sources such as probes for STM and AFM, display elements and displays, a method of producing the same aggregate, a film of the acute tip, multi-wall carbon nanotube radial aggregate and a method of producing the same film.